Enterprise Rent-A-Car lied about whether cars were available to avoid honoring a $40 certificate sold by Discover Card. This happened online when I was trying to rent a car for pickup at the Enterprise location in the MKE Milwaukee, Wisconsin

Enterprise's website said they had cars.  Discover Card said that I could redeem $20 of cashback bonus and receive a $40 certificate for use at Enterprise, instead of receiving $20 cash.  This seemed like a good deal, but it was not.  On Discover's site, I found the code (DISCBB) to tell Enterprise's website that I am using one of the certificates ( https://www.discovercard.com/cardmembersvcs/rewards/app/browsePartner?modeCode=CP092 ), and it says that "No black out dates apply" ( https://www.discovercard.com/cardmembersvcs/rewards/app/showAllPartnersTerms ). However, when I try to reserve online with the code, it says that there are no cars; if I then start another browser window and do not mention the certificate, it says that there are cars. So they do have cars, just not for anyone using the $20/$40 thing. I though of calling on the phone, confirming that a car was available, and then saying, "oh wait, I have this certificate", but the rules for the Enterprise certificate require you to use a special phone number and say that you cannot use it at the regular phone number. So they know as soon as your dial. Clever.